Deviant Artist Midnight Angel put together an arm blade stylized after the ones used in the Assassin's Creed video game series. Using a drawer opening mechanism, a spring for a screen door, and a throwing knife, the weapon works surprisingly smoothly. The full instructions are on Angel's page, and you can see it in action below.

When a door broke out of our kitchen we created a light, colorful and comfortable alternative out of plastic bags. (translation: Anna shows us how to open our new door in the kitchen with the automatic cable linkage)